Polarization-dependent loss (PDL), i.e. insertion loss dependent upon the state of polarization of the input light, is particularly undesirable in optical detectors. Various ways have been proposed to render an optical detector insensitive to the state of polarization of the input light. For example, it has been proposed to interpose in front of the optical detector an optical element which exhibits polarization-dependent loss approximately equal to that of the optical detector and orient the optical element and optical detector so that their polarization-dependent losses cancel. In theory, this technique should be capable of excellent results. In practice, however, differing characteristics of the optical element and optical detector make it difficult to match the amounts of polarization-dependent loss and orient the optical element to achieve the required cancellation over a wide wavelength and temperature range.
An alternative proposal involves interposing in front of the optical detector an optical element, such as Erbium doped optical fiber, which absorbs the polarized input light and spontaneously emits unpolarized light, at another wavelength, which is captured by the detector. Although this technique is capable of reducing the polarization dependence effects at the optical detector to very low levels, it requires restriction of the range of wavelengths of the input light and also results in low optical efficiency. Also, in order to make a low PDL measurement of the input power, the coupling of the input light into the Erbium doped fiber must be made in a manner which has low polarization dependence. These requirements limit the number of applications for this technique.